In fluid pressure operated railway brake systems, there often arises a need for converting an electrically derived signal into a fluid pressure signal suitable for direct use with the fluid pressure equipment. Conventional electro/pneumatic relay valves of the well-known self-lapping type are known to have the ability to reliably and economically provide this desired signal conversion, but not without non-linearity. This non-linearity is attributed to unavoidable properties characteristic of the magnetic circuitry on which the electro-pneumatic operation is predicated and is a limiting factor of considerable importance. In FIG. 1 of the drawings is shown a characteristic magnetic force versus coil current curve illustrating this undesirable non-linearity, which exists for the most part in the lower region of the curve, becoming substantially rectilinear near the upper region of the curve.
From the foregoing, it becomes a rather obvious solution where precise control is desired to either avoid operation of the relay valve in the non-linear region of the characteristic curve or else modify the relay valve output in the non-linear region so as to exhibit a linear characteristic. In accordance with the latter solution, conventional techniques for linearizing the curvilinear portion of the characteristic curve involves the use of a constant-current controlling amplifier for controlling the input current to the relay valve electro-magnetic coil. Such techniques, however, require complicated control circuitry to make the degree of amplification vary inversely with the slope of the characteristic curve in the non-linear region and do not justify the expense involved relative to the degree of linearity obtained.